A switching regulator as discussed in Japanese Laid-Open Patent Publication No. 11-155281 has an error amplifier which receives a reference voltage and an observation voltage obtained by subjecting an output voltage to voltage division and amplifies a difference voltage thereof. An output of the error amplifier is compared with an oscillator signal and then, a switching operation is carried out in a PWM. Oscillation frequency is changed in accordance with the size of the load, allowing for improved efficiency in case of light load.
In Japanese Laid-Open Patent Publication No. 11-155281, while an oscillation frequency of an oscillator signal charges in accordance with the size of a load, this operation corresponds to a steady state. For example, the number of switching operations per unit of time is reduced by reducing the oscillation frequency in the case of a light load, as compared to the case of a heavy load, which reduces power consumption. This configuration improves efficiency. Thus, no fluctuations in the output voltage in a transient state in case of a sudden change in the load are disclosed or suggested.
As illustrated in the Japanese Laid-Open Patent Application No. 11-155281, the DC/DC converter has an error amplifier which amplifies a difference voltage between a reference voltage and an observation voltage and changes a switching duty ratio in response to the amplification result. Thus, the fluctuations in the output voltage are recovered in response to transient fluctuations in the output voltage triggered by sudden changes in the load.
For example, the error amplifier has a circuit for phase compensation which is provided between an input and an output terminal thereof. The phase compensation circuit is made up of a resistive element, a capacitive element and the like. In the error amplifier, the output signal responds to precipitous fluctuations in the input signal with a limited time lag through the phase compensation circuit. This ensures a stable operation in the system.
Accordingly, a fixed period of time passes from an operation of inputting fluctuations in the observation voltage triggered by fluctuations in the output voltage to the error amplifier until an operation of amplifying a difference voltage between this observation voltage and the reference voltage in the error amplifier and outputting the result. Although the period of time from when the switching duty ratio is determined in accordance with the comparison result between the output signal and the oscillator signal of the error amplifier until the switching operation is carried out corresponds to a short period of time, transient fluctuations in the output voltage may not be sufficiently suppressed by the response lag in the error amplifier. Thus, the output voltage may temporarily fluctuate in a transient state, such as in the case of a sudden change in the load.
In this case, to suppress fluctuations in the output voltage, an output capacitor which has a sufficient capacitance value is preferably provided. This may lead to an increase in the size of the mounting components and the number of mounting components, resulting in a larger mounting area.